Lysosomal storage disorders represent a group of at least 50 genetically distinct, biochemically related, inherited diseases. Individually, these disorders are considered rare, although high prevalence values have been reported in some populations. These disorders are devastating for individuals and their families and result in considerable use of resources from health care systems, however, the magnitude of the problem is not well defined. Included amongst these lysosomal storage disorders are mutations or other disruptions of thioesterases, which result in the accumulation of posttranslationally lipid-modified proteins in lysosomes.
Thioesterases are enzymes in the esterase family (members of E.C.3.1.2), which split an ester (specifically at a thiol group) into an acid and alcohol, in the presence of water. Examples of human thioesterases include acetyl-coA hydrolase, palmitoyl-CoA hydrolase, succinyl-CoA hydrolase, formyl-CoA hydrolase, acyl-CoA hydrolase, and ubiquitin thiolesterase and human genes encoding thiol esterases include: ACOT1, ACOT2, ACOT4, ACOT6, ACOT7, ACOT8, ACOT9, ACOT11 (STARD14), ACOT12 (STARD15), OLAH, APT1, APT2, PPT1, PPT2, THEM2 (ACOT13), THEM4, THEM4P1, and THEM5.
An example of a lysosomal storage disorder resulting from one or more mutations in a thioesterase is infantile neuronal ceroid lipofuscinosis (INCL), which is a lethal childhood neurodegenerative storage disorder caused by palmitoyl protein thioesterase-1 (PPT1) gene mutations. Palmitoylation is a posttranslational modification in which a 16-carbon fatty acid, palmitate, is attached to specific cysteine residues in polypeptides via thioester linkage. PPT1 cleaves thioester linkages in S-acylated proteins facilitating degradation or recycling and its deficiency leads to lysosomal storage of these proteins causing INCL pathogenesis, as depicted in FIG. 1. Currently, there is no effective treatment for INCL.
As an example of cancers associated with thioesterase deficiencies, Ras is mutated in cancer more frequently than any other oncogene. Hence, Ras has been a focus for the development of rationally designed anti-cancer drugs, yet to date none have been successfully developed. Posttranslational lipid-modification of Ras proteins is essential for Ras membrane association and transformation. The differences in the four Ras isoforms, N-Ras, H-Ras, K-Ras4A and KRas4B reside in the C-terminal region referred to as the hypervariable region (HVR), which is modified by posttranslational lipid-modifications. H-Ras is modified by two cysteine palmitoylations and one cysteine farnesylation, whereas N-Ras and K-Ras(A) are modified by one cysteine palmitoylation and one cysteine farnesylation. In contrast, K-Ras(B) is not palmitoylated. Reversible palmitoylations of H- and N-Ras GTPases control their membrane attachment and specific localization on the plasma membrane and the Golgi. Proper steady state localization requires a dynamic cycle of palmitoylation on the Golgi, which redirects Ras to the plasma membrane, and ubiquitous depalmitoylation to counteract spontaneous nonspecific distribution over cellular endo membranes. Disruption of this dynamic cycle results in a reduction of Ras localization on the Golgi and the plasma membrane, due to random redistribution to endo membranes, indicating that inhibitors of palmitoylation as well as enzymatic depalmitoylation alter the steady state localization of Ras GTPases and thus Ras signaling.
Thus, thioesterases present a compelling therapeutic target for the prevention and treatment of thioesterase deficiency disorders such as lysosomal storage disorders including INCL, and cancers associated with Ras localization and activation, and there exists a need for effective methods of inhibiting Ras GTPases for the treatment and prevention of these thioesterase deficiency disorders.